External Forces
Figure 23a shows a two-story building subjected to externally applied dynamic forces pi(t) and p2(t) at the first and second floors, respectively. Under the influence of these forces, the structure is displaced in the lateral direction. At any instant of time, the displaced configuration of the structure can be spec• ified by the displacements ui(t) and u2(t) of the first and second floors, respectively.
The various forces acting on the floor masses are shown in the free-body diagrams of Fig. 23b. These include the external forces p.It), the inertia forces fIJ, and the elastic resisting forces f SJ• The inertia forces act to the left, opposite to the direction of positive acceleration. The elastic resisting forces are shown act• ing to the left, opposite to the direction of positive deformation. At each instant of time, each mass of the structure is in equilib• rium under the action of these forces at that time. From the free• body diagrams, this condition of dynamic equilibrium for the first floor mass